Typical EL lamps are applied in frame systems. Thus, the size of the EL lamp can be 2,500 square inches or more and the applied voltage can be between 100-350 VAC or even higher. If people touch the electrode of EL lamp, current may flow through the human body to ground, thus possibly causing physical injury.
It has long been known to apply a Ground Fault Interrupt (GFI) circuit to the ballast for fluorescent lamps. A conventional GFI circuit uses a current sensor to measure unbalanced current between input live and neutral. Most of the ballasts are non-isolated circuits. These GFI circuits can be applied to the non-isolated ballast for an EL lamp. Thus, when a person touches the electrode of EL lamp, the conventional GFI circuit will shut down the ballast. However, when EL lamp is put into a grounded lighting frame or fixture, there is a large parasitic capacitance (a few nF) between the fixture and EL lamp, thus resulting in high leakage current from the EL lamp to ground. This stray leakage current will trip the GFI circuit and shut down the ballast. Hence, the conventional GFI circuits cannot accurately discriminate between stray leakage current and the leakage current that occurs due to a true fault condition.
In an isolated ballast, line input is isolated from an output high voltage terminal. Thus, an isolated ballast could prevent stray leakage current flow to a grounded fixture since there is no return current path back to a secondary of the ballast. An isolated ballast with a GFI circuit could detect leakage current when a person touches the electrode of an EL lamp. However, when a person replaces an EL lamp, if the lighting fixture of the EL lamp is floating, there is still a leakage current coupled to the lighting fixture which can flow to the human body and back to ballast secondary output. The magnitude of the coupled leakage current depends on parasitic capacitance between the El lamp and the fixture, and at times can be up to 10 mA. Thus, even with an isolated ballast, there is still a risk of getting shocked when replacing such an EL lamp.
An EL lamp with a ground shield and an isolated ballast could solve the above-mentioned problem, since there is no parasitic capacitance from the EL lamp to the lighting fixture. However, if an isolated ballast is connected with a ground shield of the EL lamp and if the EL lamp is destroyed by a metal tip, if a person touches the defective part, leakage current will flow through the human body to the ground shield and couple back to the EL lamp. This shock current could be as high as 60 mA and depends on the parasitic capacitance between the ground shield and the rear electrode of EL lamp.
Thus, there is a need to provide a new safety circuit for an EL lamp to protect a user when replacing an EL lamp and when touching a defective EL lamp.